In progmodes/project.el, I believe the face reference, below, needs a quote.
(defun project-mode-line-format ()
"Compose the project mode-line."
...
(propertize
(project-name project)
'face project-mode-line-face
'face 'project-mode-line-face ; should be quoted?
...
'local-map project-mode-line-map)))))
I hope I got that right. I've been using a custom project-mode-line-format and custom project-mode-line-face which is how I noticed this.
Stephane